Photoconductive composition and article



United States Patent ()fifice 3,345,162 Patented Oct. 3, 1967 3,345,162 PHOTOQONDUCTIVE COMPGSITION AND ARTICLE Samuel B. McFarlane, Jn, Summit, Daniel J. Cal-lick, Berkeley Heights, and John Ostoclr, Middletown, NJ., assignors to Sun Chemical Corporation, New York, N.Y., a corporation of Delaware No Drawing. Filed June 17, 1963, Ser. No. 288,486 2 Claims. (Cl. 961.8)

This invention relates to novel coating compositions, electrophotographic recording members and to processes of producing such members.

Electrostatic printing processes are well known in the art (see R.C.A. Review 15 (1954), pages 462-468, French patent specification 1,113,933; US. patent specifications 2,735,784, 2,735,785, 2,727,808; German patent specification 958,355). It is known to produce photoconductive materials for such processes by casting light-sensitive or photoconductive layers on to suitable supports, from solutions of layer-forming plastics in organic solvents to which are added photoconductive substances. The binder used in such cases must possess an electric specific resistance which is greater than that of the photoconductive substance and greater than the specific resistance of the support.

An electrostatic printing process is that type of process for producing a visible record, reproduction or copy which includes as an intermediate step, converting a light image or electric signal into an electrostatic charge pattern on an electrically-insulating layer. The process may also include the conversion of the charge pattern into a visible image which may be a substantially faithful reproduction of an original except that it may be of a different size, color or contrast range.

A typical electrostatic printing process may include preparing an electrophotographic recording member, for example, by coating a surface of a backing with a photoconducting insulating material such as selenium, anthracene, or zinc oxide dispersed in an electrically-insulating, film-forming, water insoluble vehicle such as silicone resin. An overall electrostatic charge is produced on the surface of the photoconducting material and a light image is focused on the charged surface, discharging the portions irradiated by the light rays, while leaving the remainder of the surface in a charged condition thereby forming an electrostatic image. The electrostatic image is rendered visible by applying a developer powder which is held electrostatically to the charged areas of the sheet. The powder image thus formed may be fixed directly to the photoconductive coating or it may be transferred to another surface upon which the reproduced image may be desired and then fixed thereon.

According to a prior art process, the recording member for electrostatic printing is prepared by first mixing the finely-divided photoconductor and the electrically-insulating, film-forming, Water-insoluble vehicle with a volatile organic solvent for the vehicle, for example, by mixing zinc oxide and a silicone resin with toluene. This mixture is coated on the backing member by any standard coating procedure, dried and then cut or punched to a desired size.

Electrophotographic recording members have been made by coating an electrically-conducting or semi-conducting substrate, usually paper, with a solution of a resin such as a silicone resin or a polyvinyl acetate resin in an organic solvent such as toluene, xylene or mixtures of these solvents. The finely-divided photoconductor is suspended in this solution. A preferred photoconductor is zinc oxide, such, for example, as the photoconductive zinc oxide sold by the New Jersey Zinc Company as Florence Green Seal 8. Other photoconductors such as the oxides of antimony, aluminum, bismuth, cadmium, mercury, molybdenum, and lead; the iodides, selenides, sulfides or tellurides of these metals including zinc; selenium; arsenic trisulfide; lead chromate and cadmium arsenside have been suggested. The resinous vehicle forms an electrically-insulating binder for the photoconductor.

An object of this invention is to provide improved electrostatic printing processes and improved recording members therefor.

A further object is to provide improved compositions for producing recording members for electrostatic printmg.

Another object of this invention is to provide recording members for electrostatic printing which give the desired response without excessive discoloration of the surface of the members.

Another object is to provide recording members for electrostatic printing having high charge storage capabilities.

Another object is to provide recording members for electrostatic printing which, in addition, provide exceedingly good black density and contrast on development.

Generally, the recording members of this invention comprise a photoconducting layer including a major proportion of a finely-divided photoconductor, such as photoconductive zinc oxide, dispersed in a minor proportion of an electrically-insulating film-forming vehicle including a short oil alkyd resin mixed with an acrylic polymer. The photoconducting layer may be self-supporting but is preferably supported on a backing such as paper and said layer is preferably insolubilized, as by polymerization, prior to use in electrostatic printing.

The invention further includes coating compositions for coating substrates such as paper for providing a photoconductive layer thereto. These novel compositions comprise mixtures of short oil alkyd resins with acrylic polymers or melamine-formaldehyde condensates modified with acrylic polymers and can also contain suitable solvents to render the composition applicable as a coating and photographic sensitizers for obtaining the particular photographic response desired.

The invention further includes electrostatic printing processes comprising the steps of producing an electrostatic charge image corresponding to an applied light image on said recording members, developing said electrostatic image with a finely-divided developer substance and then fixing said developed image substantially in situ.

The foregoing and other objects and advantages will be more fully understood from the following detailed description.

Heretofore, the prior art recording members as described above lacked color density, e.g., black density, and contrast after development or, in those cases where good color density was obtained, the recording member underment extreme discoloration on developing.

It has been surprisingly discovered that recording members which provide good color density and contrast without discoloration are obtained by employing, as the electrically-insulating, film-forming vehicle for the photoconductor, a mixture of a short oil alkyd resin and an acrylic polymer or copolymer. This color density and contrast are not obtainable by employing the short oil alkyd resin alone or the acrylic copolymer alone, thus indicating a synergistic reaction between said alkyd resin and the acrylic polymer or copolymer.

The alkyd resins employed in this invention are the well known short oil alkyd resins, such as saturated fatty oil-modified glyceryl-phthalate, or glyceryl-isophthalate, alkyds wherein the modifying oil is any saturated or non-yellowing unsaturated fatty oil such as cocnut oil, refined tall oil, soya oil, safllower oil, etc., or the fatty acid-modified glyceryl-phthalate, or glyceryl-isophthalate, alkyds wherein the modifying acid is lauric acid, pelargonic acid, 2-ethylhexoic acid, etc. The short oil alkyd resins which can be employed are further characterized by a viscosity in the range of Z to Z at 60% solids, an acid number in the range of 4 to 15 an and oil content of 20 to 40 weight percent. Longer oil alkyd can be used but best results are obtained in the short oil range.

The acrylic polymers which are employed in this invention include lower alkyl (i.e., l to 4 carbon atoms) methacrylate polymers such as poly(isobutyl methacrylate), poly(n-butyl methacrylate), lower alkyl acrylate polymers and copolymers of acrylic esters and/ or methacrylic esters, e.g. copolymers of isobutyl methacrylate, n-butyl methacrylate with other methacrylic or acrylic esters. In place of lower alkyl methacrylate polymers there can be used melamine-formaldehyde condensates which are modified with acrylic copolymers such as acryloid ATSO. Other modifying copolymers include styrene-butadiene coplymers, vinyl acetate-ethylene copolymers, acrylonitrile-butadiene coplymers and the like. The amounts of modifying resin employed vary in the range of 10 to 40 weight parts of said copolymer per weight part of acrylic polymer or copolymer.

Almost any powdered photoconductor having sufi'iciently high values of photoconductivity may be used in the coating composition, for example, the photoconduc tive oxides, sulphides, selenides, tellurides, and iodides of cadmium, mercury, antimony, bismuth thallium molybdenum, aluminum, lead or zinc. In addition, arsenic triulsfide, cadmium arsenide, lead chromate or selenium may be used. It is preferable for the photoconductor to have a high electrical resistivity in darkness. Mixtures of one or more photoconductors may be used.

The particular photoconductor utilized determines the spectral response of the recording member. The color of the photoconductor indicates approximately the location of the absorption edge of the photoconductor and of the recording member. Most photoconductors absorb light in the shorter wavelengths. When longer wavelengths are used, a value is reached where the absorption drops off sharply and the photoconductor ceases to absorb radiation. This value is called the absorption edge of the material. It is of particular advantage that by making a proper selection of the photoconductor one may obtain a printing base with any desired light absorption characteristic and thereby any desired spectral sensitivity. For example, thallium iodide has a peak response around 4130 A. Silver sulphide has a peak response around 13,500 A., while other photoconductors may have their peak response at other wavelengths in the electro magnetic spectrum and over a narrow or wide band of frequencies.

Any suitable solvent for the short oil modified alkyd resin and the acrylic polymer or copolymer can be employed in the novel coating compositions. The function of the solvent is primarily to adjust viscosity and to permit ease of handling when the compositions are employed to coat the base substrate or support, e.g., paper. Typical solvents include especially the aromatic solvents, such as, toluene, xylene, benzene and the like.

Any suitable base substrate or support may be employed, such as, paper, metal, plastic material, cellophane, metallic sheet material, such as copper, aluminum or brass foil or sheet, or a mineral sheet material, such as glass or mica sheet. The substrate or support may be in any desired shape or configuration. It is preferred, although not necessary, that the substrate have a higher electrical conductivity than the final photoconducting coating. The substrate or support should have sufficient tensile strength for the intended use of the final recording member and should be resistant to the solvent employed in the coating composition. In certain instances, it will be desirable to pre-coat the substrate, especially when fibrous or absorbent substrates such as paper are employed. The pro-coating can be employed to improve the tensile strength and solvent-holdout and should be relatively electrically conductive as compared with the photoconductive coating having a lateral resistivity of about 1X10 to 1x10 ohms per square centimeters. Any suitable pro-coating composition can be employed, for example, cellulose acetate, poly(vinylidene chloride), poly(vinyl acetate), poly(vinyl alcohol) or mixtures thereof. If desired, the electrical conductivity of the pre-coating composition can be increased by the addition of electrolytes. Substrates such as paper suitable for electrophotographic recording supports are generally available commercially.

In addition, the spectral sensitivity of the photoconductive coating of this invention can be altered by the addition of sensitizing dyes or similar substances, for example, Rose Bengal or Rhodamine B. Also, various dyes, optical brighteners and the like can be employed in the novel compositions for providing various color effects to the substrate to which the coating is applied. In addition, suitable fillers, plasticizers and waxes can be added to the novel coating compositions to provide special effects to the coated substrate.

The proportion of photoconductor to film-forming vehicle in the novel compositions of this invention can be varied within the range of 3 to 10 weight parts of photoconductor, preferably 4 to 6 weight parts, per weight part of film-forming vehicle including the short oil alkyl resin and acrylic polymer or copolymer. The amount of acrylic polymer can be varied from 0.2 to 5 weight parts, preferably 1 to 3 weight parts, per weight part of short oil alkyd resin. The amount of solvent is not narrowly critical and is primarily dependent upon the desired coating procedure, viscosity, thickness of coating, and the like. The amount of sensitizers also is not narrowly critical and can vary between 0.01 to 0.05 weight part of sensitizer per weight parts of photoconductor.

The coating compositions of this invention can be applied to the substrate in any desired manner, such as through the use of a reverse roll coater or other similar machinery and techniques. After application, the coated substrate is dried. The thickness of the coating applied can vary over a large range, although thicknesses of 0.2 to 1 mil of the dried coating provide the unexpected advantages of this invention. Thicknesses outside of this range can also be employed, if desired.

The following examples are presented:

Examples 1 through 9 In these examples, zinc oxide (Green Seal 8, New Jersey Zinc Company) is dispersed in an organic solvent solution containing coconut oil-modified glyceryl-phthalate alkyd resin having a viscosity of about Z at 60% solids, an acid number of about 5.0, and a coconut oil content of about 25 percent (Duraplex ND7 6, Rohm and Haas Company) and a polyisobutyl methacrylate resin having an intrinsic viscosity of 0.66 (Lucite 2045-Du Pont) which has been dissolved in toluene. The weight ratio (solids) of the alkyd to acrylic resin is 1:3. The resulting dispersion is coated onto a semi-conductive paper by a reverse roll coater (Contracoater by the Black- Clawson Company, Fulton, NY.) and dried to provide a coating of 0.5-1 mil thickness.

The amounts of zinc oxide, alkyd resin, polyisobutyl methacrylate and toluene solvent .are listed in Table I below. The electrostatic recording members thus prepared had a high charge storage capability and provided excellent color density and contrast on development. The relative sensitivities of all recording members produced in these examples were well above acceptable levels.

TABLE I Wt. Pts. Poly- Wt. Pts. Alkyd Wt. Pts. Zinc Wt. Ratio Relative Sur- Isobutyl Meth- Resin Duraplex Wt. Pts. Oxide (Green Polyisobutyl face Potential acrylate (100% ND-76 (60% Toulene Seal #8) Methacrylate (volts) at R.IE[. Print Density Solids) Solids) to Alkyd 30% Direct Reading 60.0 20. 424. 0 360. 0 :1 34 Good. 49. 9 20.0 366.6 312. 5 4:1 34 D0. 54. 0 30. 0 420.0 360. 0 3:1 40 Excellent. 22.8 19.0 196. 1 169. 5 2:1 45 D0. 22. 5 37. 5 205. 0 275. 0 1:1 46 D0. 24. 9 83. 0 415. 0 373. 5 1:2 44 Do. 12.5 62. 5 275. 0 250. 0 1:3 44 Do. 12.5 83. 3 341. 6 312.4 1:4 44 Do. 10. 0 83.3 326.4 300. 0 1:5 32 Good Measured by using a thin tungsten w ire electrode 2" long, spaced approximately from the paper surface. This wire was connected by a shielded lead to an electrometer voltmeter. The actual surface voltage can be obtained by multiplying the readings by a calibration constant of approximately 12.

Examples 10 through 13 The same type of zinc oxide employed in the previous examples was dispersed in solutions of the same type of short oil alkyd resin and polyisobutyl methacrylate resin as employed in the previous examples. The resulting dispersions were coated in the same manner to provide photoconductive members having a photoconductive layer of about 1 mil thickness. The electrostatic recording members thus prepared had a high charge storage capability and provided excellent color density and contrast on development. The relative sensitivities of all recording members produced in these examples were well above accepttrostatic printing comprising a backing sheet coated with a photoconducting composition containing photoconducting zinc oxide and a mixed resin comprising a coconut oil modified alkyd resin and a lower alkyl methacrylate polymer in the amounts of 0.2 to 5 weight parts of said methacrylate polymer per weight part of said alkyd resin and 3 to 10 weight parts of zinc oxide per weight part of said mixed resin.

2. A photoconducting composition for use in electrostatic printing comprising photoconducting zinc oxide and a mixed resin comprising a coconut oil modified glycerylphthalate alkyd resin and lower alkyl methacrylate resin able levels in the amounts of 0.2 to 5 we1ght parts of said meth- TABLE II Wt. Pts. Wt. Pts. Wt. Pts. 't. Ratio Relative Sur- Poly'Isobutyl Alkyd Resin Wt. Pts. Zinc Oxide Zinc Oxide iace* Poten ial Rel. Sens. Print Methaerylate Duraplex Toluene (Green Seal to the (volts) R.HJ30% (Speed) Density (100% Solids) N1?s76d((;0% #8) Resin Binders Direct Reading 30 681 630 10 :1 36 12% Excellent 45 30 492 441 7: 1 38 12 Do. 45 30 366 315 5:1 42 12 D0. 45 30 303 252 4:1 42 11% Do. 45 30 240 189 3:1 34 11 Do.

*Measured by using a thin tungsten wire electrode 2 lon face. This wire was connected by a shielded lead to an electro obtained by multiplying the readings by The photoconductive members according to this invention are used in the usual manner. The materials are first made sensitive to light by giving them an electrostatic charge on the coating side in the dark, for instance by means of a corona discharge. The material is then exposed by any of the conventional photographic procedures. The latent image obtained is developed by applying a pigmented resin powder, for example, containing, in addition to the pigment, a heat-activated resin binder and electrically conductive particles, carrying an electrostatic charge which is opposite to that of the photoconductive layer. The powder image produced is then fixed by melting the resin powder.

What is claimed is:

1, An article of manufacture adapted for use in elecg, spaced approximately /1 meter voltmeter. The actual surface voltage can be a calibration constant of approximately 12.

from the paper suracrylate resin per weight part of said alkyd resin and 3 to 10 weight parts of zinc oxide per weight part of said mixed resin.

References Cited UNITED STATES PATENTS 2,872,422 2/1959 Rolle et a1. 260-16 3,041,465 6/1962 Sus et al. 961 3,121,006 2/1964- Middleton et al. 961 3,128,260 4/1964 Langstroth 260 -22 3,240,597 3/1966 Fox 96-1.5

NORMAN G. TORCHIN, Primary Examiner.

C. E. VAN HORN. D. D. PRICE. Assistant Examiners. 

1. AN ARTICLE OF MANUFACTURE ADAPTED FOR USE IN ELECTROSATIC PRINTING COMPRISING A BACKING SHEET COATED WITH A PHOTOCONDUCTING COMPOSITION CONTAINING PHOTOCONDUCTING ZINC OXIDE AND A MIXED RESIN COMPRISING A COCONUT OIL MODIFIED ALKYD RESIN AND A LOWER ALKYL METHACRYLATE POLYMER IN THE AMOUNTS OF 0.2 TO 5 WEIGHT PARTS OF SAID METHACRYLATE POLYMER PER WEIGHT PART OF SAID ALKYD RESIN AND 3 TO 10 WEIGHT PARTS OF ZINC OXIDE PER WEIGHT PART OF SAID MIXED RESIN. 